Filter devices of the above general kind, as currently known in the prior art, generally comprise an element, a so-called particle filter, which arrests solid particles entrained by the air passing through the filter, together with an element in the form of an odour filter which consists essentially of activated carbon.
The activated carbon is in the form of porous granules which are disposed downstream of the particle filter. The major problem which then arises in such prior art filters is the difficulty of inserting and maintaining the activated carbon granules in the filter device.
It has been proposed to utilize linking particles mixed with the activated carbon granules, so as to fix the latter on the support in which they are placed, in practice the particle filter. However, the efficiency of such an odour filter does not have an optimum value, for two reasons. In the first place the presence of linking particles proportionally reduces the quantity of activated carbon granules. In addition, the linking particles are provided for the purpose of bonding the activated carbon granules on a support; but as a result of this bonding, a major proportion of the pores in the activated carbon granules are obstructed. This reduces the capacity of the activated carbon granules to retain gases and at the same time increases the loss of pressure in the odour filter.